drm/amd/powerplay: move power_dpm_force_performance_level to amdgpu_smu file

because this callback is not asic related function, so move it to top
code level to support more asic (eg: navi10)

Signed-off-by: Kevin Wang <kevin1.wang@amd.com>
Reviewed-by: Huang Rui <ray.huang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>